Adding Security as a new Ideas category

Not really. Using a tag for it is fine, and in general the ideas around it include:

* being able to move wp-admin/wp-login
* Captcha
* Two Factor Authentication

All of those are -currently- best served as plugins, so for now there's not really a need. If there are security problems, we hope people read https://codex.wordpress.org/FAQ_Security and disclose appropriately :)